1. Field of the Invention
The present invention relates to an electrophotographic image forming apparatus in which a polygon motor is used for exposure scanning for forming a latent image, and a method for controlling such an electrophotographic image forming apparatus.
2. Description of the Related Art
A polygon motor having an oil dynamic bearing is widely used as a rotary drive source for a laser scanner. This type of polygon motor has temperature dependence attributable to the viscosity of oil. A start-up time of the polygon motor is significantly longer in the case where an ambient temperature is lower than a predetermined temperature, e.g., in winter, than in the case where the ambient temperature is not lower than the predetermined temperature. The start-up time herein represents the time period from when the polygon motor starts to rotate to when it rotates stably at a speed suitable for exposure operation.
As for control of the start-up of such a polygon motor, the following technology is described in Japanese Laid-open Patent Publication No. 2001-083451. To be specific, in determining that the motor fails for a case where the rotational speed thereof does not reach a target speed even after a predetermined amount of time has elapsed, the predetermined amount of time is extended in the case of low temperatures. Further, another technology is proposed in which, as warm-up operation of a motor at the time of turning the power ON, the motor is rotated at a speed lower than a rotational speed thereof for image formation (Japanese Laid-open Patent Publication No. 2005-215174). Further, another technology is proposed in which, in the case of changing the number of revolutions of a motor, for example, changing it from the number of revolutions under the stand-by state to a first or second number of revolutions at the time of image formation, optical scanning is permitted if a certain time period determined based on the difference between the pre-change and the post-change in number of revolutions has elapsed (Japanese Patent No. 3683666).
Meanwhile, it can be conceived to feed a sheet of paper without stopping the paper from a sheet deck to a position at which a toner image is transferred. The use of such a non-stop paper feed simplifies the structure of the paper feed mechanism and the control thereof, which results in the reduction of the device cost. For the non-stop paper feed, it is necessary to set a paper feed start timing in such a manner that a sheet of paper reaches a transfer position after the state of devices including a polygon motor related to an electrophotographic process is shifted from a standby state to a state suitable for image formation. Assume that the paper feed start timing is set by using, as the reference, an operating ambient temperature under which the start-up time of the polygon motor is longest. In such a case, an inconvenience situation that a sheet of paper reaches the transfer position before the polygon motor starts up completely does not occur. However, in the case of operation under environmental conditions where the start-up time of the polygon motor is relatively short, it takes an excessively long time to completely form an image on the first page since a user gave a command to form images. Such a period of time is referred to as a “first print-out time”.